1. Field of the Invention
The present invention relates to apparatus and methods for oil and gas wells to enhance the production of subterranean wells, either open hole, cased hole, or cemented in place and more particularly to improved multizone stimulation systems.
2. Description of Related Art
Wells are drilled to a depth in order to intersect a series of formations or zones in order to produce hydrocarbons from beneath the earth. Some wells are drilled horizontally through a formation and it is desired to section the wellbore in order to achieve a better stimulation along the length of the horizontal wellbore. The drilled wells are cased and cemented to a planned depth or a portion of the well is left open hole.
Producing formations intersect with the well bore in order to create a flow path to the surface. Stimulation processes, such as fracing or acidizing are used to increase the flow of hydrocarbons through the formations. The formations may have reduced permeability due to mud and drilling damage or other formation characteristics. In order to increase the flow of hydrocarbons through the formations, it is desirable to treat the formations to increase flow area and permeability. This is done most effectively by setting either open-hole packers or cased-hole packers at intervals along the length of the wellbore. These packers isolate sections of the formations so that each section can be better treated for productivity. Between the packers is a frac port and in some cases a sliding sleeve or a casing that communicates with the formation or sometimes open hole. In order to direct a treatment fluid through a frac port and into the formation, a seat or valve may be placed above a sliding sleeve or below a frac port. A ball or plug may be dropped to land on the seat in order to direct fluid through the frac port and into the formation.
One method, furnished by PackersPlus, places a series of ball seats below the frac ports with each seat size accepting a different ball size. Smaller diameter seats are at the bottom of the completion and the seat size increases for each zone as you go up the well. For each seat size there is a ball size so the smallest ball is dropped first to clear all the larger seats until it reaches the appropriate seat. In cases where many zones are being treated, maybe as many as 20 zones, the seat diameters have to be very close. The balls that are dropped have less surface area to land on as the number of zones increase. With less seat surface to land on, the amount of pressure you can put on the ball, especially at elevated temperature, becomes less and less. This means you can't get adequate pressure to frac the zone because the ball is so weak, so the ball blows through the seat. Furthermore, the small ball seats reduce the I.D. of the production flow path which creates other problems. The small I.D. prevents re-entry of other downhole devices, i.e., plugs, running and pulling tools, shifting tools for sliding sleeves, perforating gun size (smaller guns, less penetration), and of course production rates. In order to remove the seats, a milling run is needed to mill out all the seats and any balls that remain in the well.
The size of the ball seats and related balls limits the number of zones that can be treated in a single trip. Furthermore, the balls have to be dropped from the surface for each zone and gravitated or pumped to the seats.
Another method, used by PackersPlus, U.S. Pat. No. 7,543,634 B2, places sleeves in the I.D. of the tubing string. These sleeves cover the frac ports and packers are placed above and below the frac ports. Varying sizes of balls or plugs are dropped on top of the sleeves and when pressuring down the tubing, the pressure acts on the ball and the ball forces the sleeve downward. Once again you have the restriction of the ball seats and theoretically, and most likely in practice, when the ball shifts the sleeve downward, the frac port opens and allows the force due to pressure diminish off before the sleeve is fully opened. If the ball and sleeve remain in the flow path, the flow path is restricted for the frac operation.
It would be advantageous to have a system that had no ball seats that restrict the I.D. of the tubing and to eliminate the need to spend the time and expense of milling out the ball seats, not to mention the debris created by the milling operation. Also, it would be beneficial to have a system that automatically fully opens each sliding sleeve and isolates the zone below, progressively up the well bore, before each zone is stimulated. Such a system allows stimulation of one zone at a time to achieve the maximum frac efficiency for each zone. In addition, it would be advantageous to be able to, in the future, isolate any zones by closing a sliding sleeve. For example, a single zone could be shut off if it began producing water or became a theft zone.
Furthermore, it would be greatly advantageous to eliminate the time and logistics required for dropping numerous balls into the well, one at a time, for each zone in the well to be treated. It would also be advantageous to have a multizone frac system that functioned automatically while all zones were being stimulated in order to minimize the time surface pumping equipment is setting idol between pumping zones.
Many wells are being stimulated at multiple zones through the well bore by use of composite plugs such as the “Halliburton Obsidian Frac Plug” or the “Owen Type ‘A’ Frac Plug”. A composite plug is set near, or below, a zone and then the zone is treated. Another composite plug is set in the next upper zone and that zone is treated, and so on up the well bore until multiple plugs remain in the well. The composite plugs are then drilled out which can be time consuming and expensive. The shavings from the mill operation leave trash in the well and can also plug off flow chokes at the surface. It would be advantageous to have a system that eliminated the use and drilling out of composite or millable plugs. Of course, this approach would apply to new well completions where equipment, of the present invention, could be placed into the well prior to treating.
Other well completions, such as intelligent wells, are designed to operate downhole devices by use of control lines running from the surface to various downhole devices such as packers, sleeves, valves, etc. An example of this type of system can be found in Schlumberger U.S. Pat. No. 6,817,410 B2. This patent describes use of control lines and the various devices they operate. It is obvious the use of control lines can make the completion very complicated and expensive. The present invention allows operation of some types of downhole devices possible without the use of control lines. For example, the present invention describes a timer/pressure device that could be placed both above and below a sliding sleeve, and days, months, or even years later, a sliding sleeve, or series of sliding sleeves, could be programmed to open or close.
There are other wells that sometimes require well intervention. A product called a Well Tractor, supplied by Welltec, is used to aid in shifting sliding sleeves opened or closed in long horizontal wells or highly deviated wells, sometimes in conjunction with wireline or coiled tubing operations. The present invention offers an alternate and more economical solution to functioning downhole devices in wells without well intervention.